The present invention relates to a class of peptides that are antigenic determinants identified by antibodies obtained using a virus, bacterial cell or other pathogenic agent, or a derivative of such an agent, that presents a restricted set of antigens to surveillance by the immune system. The present invention also relates to immunogens that comprise such an antigenic determinant, and to vaccines containing such an immunogen.
Development of an effective, practical vaccine against a given pathogenic agent often hinges on identifying and characterizing targets of antibody-mediated immunity against that agent. Typically, the search for such target antigens is complicated by difficulties encountered in selecting, from among a large number of antigens contained in the pathogenic agent, those that are important in eliciting a protective immune response. Even a relatively simple pathogen like a poliovirus, among the least complex of small spherical viruses, may comprise many antigens; yet only a subset of these are positioned, at one time or another during the life cycle of the virus, so as to be accessible to antibody-binding.
The problems of identifying members of this subset, and of characterizing the antigenic determinants they present, is exacerbated in the case of more complicated pathogens, such as trypanosome and malaria parasites, by a larger number of antigens and a more diverse range of life forms. For example, within the human host, the life cycle of the malaria parasite encompasses four different stages (sporozoite, exoerythrocytic, asexual erythrocytic, and gametocytic) and numerous forms, each morphologically and antigenically distinct. There is also antigenic diversity in natural parasite populations, including expression of different isozymes, different sensitivities to antimalarial drugs, and expression of alternative antigenic determinants ("epitopes") on particular malarial proteins. Finally, malaria parasites may express new antigenic phenotypes through mutation or through recombination within a gene.
Existing methods for identifying particular antigens as prime immunological targets derive from one of several approaches to epitope-mapping. By one approach, monoclonal antibodies can be used to compare the relative rate of chemical modification of residues of a protein antigen in the presence or absence of a specific monoclonal antibody, thereby permitting one to deduce the location of the epitope from the differential chemical reactivity of amino acid side chains. See, e.g., Burnens et al, Science 235: 780 (1987). Alternatively, a monoclonal antibody can also be used to identify synthetic peptides that are putative epitopes or functional analogs thereof. Ballou et al., Science 228: 996 (1985). Additionally, monoclonal escape mutants can be characterized by means of monoclonal antibodies. Hogle et al., Sci. Amer. 256: 42 (1987).
These applications, while useful, present the disadvantage that prerequisite functional monoclonal antibodies are rarely available and are not readily prepared for many antigens. Alternatively, synthetic peptides can be used to induce functional monospecific antibodies (Ballou et al., 1985). By exploiting the ability of intact infectious organisms to adsorb out peptide-specific antibody from neutralizing antiserum, synthetic peptides can also be used to identify epitope specificities recognized by functional antibodies in vivo. Both of these approaches typically require the synthesis of a vast number of different peptides, however, and especially when epitopes are formed by amino acids that are noncontiguous in the antigen.